Polyester-polyurethane hybrid resins are well-known in the art of thermoset molding compositions. The term hybrid describes a single, new type of polymer that is formed by the incorporation of the chemical groups and the properties of two different polymers, namely polyurethanes and unsaturated polyesters. Hybrid resins build molecular weight and toughness as they cure through the urethane chain-extension reaction, which occurs between the hydroxyl end groups on the polyester polyol and the isocyanate groups. Crosslinking occurs between the unsaturation in the polyester backbone and the styrene monomer, adding stiffness and thermal resistance. Thus, a unique blend of properties is obtained that cannot be achieved with either type of polymer alone. The hybrid resins are normally tougher than non-hybrid polyesters and stronger, stiffer and less expensive than polyurethanes. Polyester-polyurethane hybrid resins typically comprise a hydroxyl-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer, such as styrene, and a polyisocyanate. Polyester-polyurethane hybrid resins can be easily adapted to many common thermoset molding techniques employed in both the polyurethane and unsaturated polyester industries. Such hybrid resins are generally supplied as a two component system having an Aside and a B-side. The Aside typically contains the polyisocyanate and a free radical initiator, while the Bside typically contains the hydroxyl-terminated unsaturated polyester polyol/styrene solution and, optionally, fillers and/or additives.
One key problem that occurs during the hybrid cure is resin shrinkage which results in dimensional stability problems, such as warpage. Unacceptable surface appearances such as waviness or roughness results when hybrids are reinforced with fibers, such as glass. The resin shrinks around the glass fibers, allowing the fibers to show through the surface of the molded article. This phenomenon is commonly termed glass print-through. It is desirable to reduce the shrinkage and improve the surface appearance (profile) of molded articles manufactured from hybrid resins.
U.S. Pat. No. 4,822,849, teaches reducing the shrinkage of hybrid resins by reducing both the styrene level and unsaturation level within the hybrid. Lower shrinkage is achieved by reducing the crosslink density, but this may lead to reduced thermal properties of the hybrid resin. U.S. Pat. No. 4,280,979, also describes the preparation of unsaturated polyester polyols, which can be reacted with a polyisocyanate and a polymerizable ethylenically unsaturated monomer to produce polyurethane/vinyl copolymers. Both patents are incorporated herein by reference.
Low profile additives (LPA's) have been added to unsaturated polyester resins to control shrinkage and improve dimensional stability and surface smoothness (profile). The LPA tends to phase separate from the polyester during cure, resulting in thermoplastic domains that induce stresses within the system. These stresses lead to the formation of internal imperfections, such as microcracks and microvoids, in molded products containing the LPA. The internal imperfections are beneficial because they reduce the amount of shrinkage that occurs during cure. Typical LPA-modified polyesters may contain up to 60 weight percent styrene monomer, and typically have high levels of unsaturation (greater than 5.5 moles unsaturation per kilogram of polyol). When there is more than one mole of styrene per equivalent of unsaturation in the polyester, the product tends to have high shrinkages; however, this shrinkage is counterbalanced by the micro-imperfections developed during the phase separation of the thermoplastic from the thermoset.
Severe glass print-through occurs in the typical conventional composite hybrid molding compositions. The failure of conventional LPA technology in commercial hybrid resins has been attributed to several factors. First, low levels of unsaturation in the polyester contribute to slow reactivity of the hybrid system: fast reactivity is considered to be one of the keys to achieving effective low profiling behavior. Also, the polyurethane reaction is considered to be slower than the unsaturated polyester crosslinking reaction; consequently, hybrid reactivity was always assumed to be significantly less than that for the corresponding polyester system. Second, the hybrid resin has a high matrix toughness compared to the unsaturated (non-hybrid) polyester resins because of the polyurethane component; therefore, the hybrid resin will not form microcracks or craze as easily. This cracking is essential for reducing shrinkage in a low profile system. And third, the low profile additive is highly soluble in the isocyanate component and, thus, is less likely to phase separate and low profile during the cure. To date, there have been no reports of observing effective low profiling behavior in hybrid resins.
Typical commercial hybrids possess low levels of unsaturation in the polyester (less than 4.0 moles/Kg) which typically results in slow reactivity. High levels of unsaturation and fast reactivity are generally believed to be necessary for achieving effective phase separation of the low profile additive. The unsaturation level in a polyol can be increased by substituting an unsaturated anhydride or acid for the saturated anhydride or acid in the polyol preparation. The reactivity of the polyol can also be increased by increasing the concentration of the fumarate (trans) isomer of unsaturation compared to the maleate (cis) isomer.
We have found that urethane catalysis of low profile-modified unsaturated polyester-polyurethane hybrid compositions yields improved processability and provides improved surface appearance (profile) in molded articles that exceeds the processing and surface appearance properties offered by traditional low profile-modified, non-hybrid unsaturated polyester resin systems. This advantage offered by the urethane-catalyzed, hybrid systems is unique and unexpected since it was previously thought that hybrids could not yield effective low profiling behavior. The rate of the crosslinking reaction has been assumed to be critical for achieving the phase separation of the low profile additive. The polyurethane reaction is predominantly a chain extension (non-crosslinking) process and was assumed to prohibit the low profiling behavior in hybrid resins. We have observed, however, that accelerating the polyurethane reaction is critical for achieving molded parts with superior surface appearance properties.